Method for axial alignment of charged particle beam and charged particle beam system

ABSTRACT

A method for axial alignment of a charged particle beam relative to at least three stages of multipole elements and a charged particle beam system capable of making the axial alignment. Some parts of the orbit of the beam or the distributions of three astigmatic fields, or both, are simultaneously translated in a direction perpendicular to the optical axis such that astigmatisms of the same order and same type due to axial deviations between successive ones of the astigmatic fields cancel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending United Statespatent application Ser. No. 13/297,628 filed Nov. 16, 2011, which claimspriority to Japanese Patent Application No. 2010-255836 filed Nov. 16,2010, which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making axial alignment of acharged particle beam with at least three stages of multipole elements.The invention also relates to a charged particle beam system capable ofmaking this axial alignment.

2. Description of Related Art

In a charged particle beam instrument (such as a transmission electronmicroscope (TEM) or scanning transmission electron microscope (STEM)),aberration correction is an essential technique for obtaining highspatial resolution. Especially, positive spherical aberration producedby an objective lens is a typical factor to limit spatial resolution(see H. Sawada et al., Journal of Electron Microscopy, vol. 58 (2009),pp. 341-347).

Today, it is widely known that this positive spherical aberration can becorrected by the use of negative spherical aberration produced byhexapole elements or the like. A hexapole element produces a three-foldastigmatism that is a second-order aberration. In spherical aberrationcorrectors, hexapole elements of this construction are arranged in twostages such that their three-fold astigmatisms cancel out each other(see H. Rose, Optik, Vol. 85 (1990), pp. 19-24 and H. Haider et al.,Nature, vol. 392 (1998), pp. 768-769).

The aforementioned aberration correction technique is fundamentallycorrection of third-order spherical aberration. Corrections up to thefourth-order aberration can be made by axial alignment, which can beachieved using an optical system. A fifth-order spherical aberration canbe corrected if the distance between the objective lens and theaberration corrector is controlled optically. However, corrections up tostill higher orders of aberrations cannot be achieved completely. On theother hand, correction of astigmatism of the same order (fifth order)(i.e., six-fold astigmatism) is not yet achieved. Therefore, in aspherical aberration corrector consisting of two stages of hexapoleelements, six-fold astigmatism is a major aberration which remains aftercorrection of third-order spherical aberration and which is a factorlimiting spatial resolution. Where six-fold astigmatism cannot becorrected, further improvement of spatial resolution cannot beanticipated.

Therefore, in the aberration corrector of JP-A-2009-054565, three stagesof three-fold fields are used to correct six-fold astigmatism. In thisaberration corrector, three-fold symmetric fields produced by the middleand rear stages of multipole elements are distributed in certainrotational relationships to the three-fold symmetric field produced bythe front stage of multipole element within the plane perpendicular tothe optical axis.

When axial alignment of an electron beam with multipole elements ismade, it is customary to deflect the beam using a deflector or the like.In the case of an imaging system aberration corrector, deflection forthe axial alignment is done while grasping residual aberration in theaberration corrector having multipole elements by tilting the beamrelative to an amorphous sample, taking an image of the sample, andsubjecting the image to Fourier transformation. In the case of thisillumination system aberration corrector, the processing is continueduntil a geometrical figure of a desired shape is obtained whilemonitoring variations in the distortion of the geometrical figure, knownas a Ronchigram, which is observed on a diffraction plane when theelectron beam is focused onto the sample. In the case of a two-stagemultipole element system, the source of appearing aberration has beenidentified and so it is relatively easy to make axial alignment of thebeam with both multipole elements.

In the case of axial alignment with three or more stages of multipoleelements, if the axial alignment with the first stage of multipoleelement produces a new aberration, it is very difficult to identifywhich of the middle and rear stages of multipole elements has broughtabout the aberration. If a further stage of multipole element is added,the difficulty increases vastly. Accordingly, a quite long time would beconsumed in making the axial alignment.

SUMMARY OF THE INVENTION

The present invention has been made to solve the foregoing problem. Itis an object of the invention to provide a method of making axialalignment of a charged particle beam with at least three stages ofmultipole elements. It is another object to provide a charged particlebeam system capable of making this axial alignment.

A first embodiment of the present invention provides a method of makingaxial alignment of a charged particle beam in a charged particle beamsystem. In this method, at least three astigmatic fields are produced.Parts of the orbit of the charged particle beam or distributions of theastigmatic fields, or both, are moved simultaneously along a directionperpendicular to the optical axis such that astigmatisms of the sameorder and same type due to axial deviations between successive ones ofthe astigmatic fields cancel out each other.

A second embodiment of the invention provides a method of making axialalignment of a charged particle beam in a charged particle beam system.In this method, at least 3 three-fold astigmatic fields are produced. Apair of transfer lenses consisting of two rotationally symmetric lensfields is formed between any adjacent two of the astigmatic fields suchthat planes on the three-fold astigmatic fields are transferred asequivalent planes to their downstream astigmatic fields. Thus, firstaxial alignments of the charged particle beam with the three-foldastigmatic fields are made simultaneously.

A third embodiment of the invention provides a method of making axialalignment of a charged particle beam in a charged particle beam system.In this method, 3 three-fold astigmatic fields are produced. Thethree-fold astigmatic fields are present on planes of multipoleelements. A pair of transfer lenses for transferring the multipoleelement planes to their downstream multipole element planes is formedbetween any adjacent two of the astigmatic fields. The intensities ofthe three-fold astigmatic fields are varied simultaneously whilemaintaining the summation of three-fold astigmatisms produced by thethree-fold astigmatic fields at zero at all times. Preferably, thedifference between the phase angles of the three-fold astigmatic fieldslocated on the most upstream side and on the most downstream side,respectively, of the 3 three-fold astigmatic fields is approximately(70.5/3)°.

A fourth embodiment of the invention provides a charged particle beamsystem having at least three stages of multipole elements for producingastigmatic fields and deflectors mounted between the multipole elements.The deflectors translate parts of the orbit of a charged particle beamsimultaneously along a direction perpendicular to an optical axis suchthat astigmatisms of the same order and same type due to axialdeviations between successive ones of the multipole elements cancel outeach other.

A fifth embodiment of the invention provides a charged particle beamsystem having at least three stages of multipole elements for producingastigmatic fields and a moving mechanism for translating the at leastthree stages of multipole elements separately in a directionperpendicular to an optical axis. The moving mechanism translates themultipole elements simultaneously such that astigmatisms of the sameorder and same type due to axial deviations between successive ones ofthe multipole elements cancel out each other.

A sixth embodiment of the present invention provides a charged particlebeam system having at least three stages of multipole elements forproducing astigmatic fields, pairs of transfer lenses mounted betweenthe multipole elements, each pair of transfer lenses producing a pair ofrotationally symmetric fields to transfer planes equivalent to planesformed by the multipole elements to their downstream multipole elements,and a moving mechanism for translating the multipole elements or thetransfer lenses, or both, separately in a direction perpendicular to anoptical axis. The moving mechanism carries out the translationssimultaneously such that astigmatisms of the same order and same typedue to axial deviations between successive ones of the multipoleelements cancel out each other.

A seventh embodiment of the present invention provides a chargedparticle beam system having at least three stages of multipole elementsfor producing three-fold astigmatic fields, pairs of transfer lensesmounted between successive ones of the multipole elements such that eachpair of transfer lenses produces a pair of rotationally symmetric fieldsto transfer planes equivalent to planes formed by the multipole elementsto their downstream multipole elements, and first deflectors mountedbetween the transfer lenses of each pair. The first deflectorssimultaneously perform deflections for making parts of the orbit of thecharged particle beam strike the optical axis obliquely.

An eighth embodiment of the invention provides a charged particle beamsystem having three stages of multipole elements for producingthree-fold astigmatic fields, first deflectors mounted upstream of themultipole elements, and transfer lenses of a pair mounted upstream anddownstream, respectively, of the first deflectors. The transfer lensesproduce a pair of rotationally symmetric fields to transfer planesequivalent to planes formed by the multipole elements to theirdownstream multipole elements. The multipole elements vary theintensities of the three-fold astigmatic fields simultaneously whilemaintaining the summation of three-fold astigmatisms produced by thethree-fold astigmatic fields at zero at all times. Preferably, thedifference between the phase angles of the three-fold astigmatic fieldsproduced respectively by the multipole elements located on the mostupstream side and on the most downstream side, respectively, of thethree stages of multipole elements is approximately (70.5/3)°.

According to the present invention, axial alignment can be made whilepaying attention to only certain aberration by simultaneouslycontrolling parts of the orbit of the charged particle beam incident onthe astigmatic fields or the astigmatic fields or the rotationallysymmetric fields acting on the charged particle beam. In other words,aberrations other than the certain aberration that is produced when theparts of the orbit, astigmatic fields, or rotationally symmetric fieldsare controlled separately do not appear. Consequently, the axialalignment is facilitated. This reduces the burden on the operator andcan contribute to a decrease in the alignment time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the principle of a method of makingaxial alignment of a charged particle beam, the method being associatedwith a first embodiment of the present invention;

FIG. 2 is a schematic diagram of an aberration corrector associated withfirst through third embodiments of the invention;

FIG. 3 is a schematic block diagram of one example of charged particlebeam system associated with embodiments of the invention;

FIG. 4 is a schematic block diagram of one example of charged particlebeam system associated with embodiments of the invention;

FIG. 5 is a schematic ray diagram of an aberration corrector associatedwith a fourth embodiment of the invention;

FIG. 6 is a schematic ray diagram of an aberration corrector associatedwith a fifth embodiment of the invention;

FIG. 7A is a diagram illustrating relations between three-foldastigmatisms in an aberration corrector associated with a sixthembodiment of the invention, as well as their variations;

FIG. 7B is a diagram similar to FIG. 7A, but in which the three-foldastigmatisms are varied;

FIG. 8A is a diagram illustrating one example of relations betweenthree-fold astigmatisms in an aberration corrector associated with thesixth embodiment of the invention, as well as their variations;

FIG. 8B is a diagram similar to FIG. 8A, but in which the three-foldastigmatisms are varied;

FIG. 8C is a diagram illustrating another example of relations betweenthree-fold astigmatisms in the aberration corrector associated with thesixth embodiment of the invention, as well as their variations; and

FIG. 8D is a diagram similar to FIG. 8C, but in which the three-foldastigmatisms are varied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A method of making axial alignment of a charged particle beam and itsprinciple are described, the method being associated with a firstembodiment of the present invention.

Referring to FIG. 1, as a comparative example for the presentembodiment, an aberration corrector 100 having two stages of multipoleelements 111 and 112 is assumed. In the corrector 100, the multipoleelements 111 and 112 are arrayed in a line along an optical axis OP.Each of the multipole elements 111 and 112 produces an N-fold astigmaticfield (where N is an integer). For convenience of illustration, it isassumed that N=3. An N-fold astigmatic field means a field in which theintensity of the produced field varies with a period of 360/N degreesaround the rotationally symmetric axis of the multipole element.

Preferably, each of the multipole elements 111 and 112 is made of ahexapole element or a dodecapole (12-pole) element. Note that nolimitations are imposed on the number of poles. The produced three-foldastigmatic field is a static electric field, a static magnetic field, ora superimposition thereof.

Each of the multipole elements 111 and 112 has a thickness oft along theoptical axis OP. The three-fold astigmatic field produced by eachmultipole element is termed the primary term of the field produced bythe multipole element. Generally, a multipole element produces fieldsowing to higher-order terms other than the primary term though they arequite weak. In an ordinary multipole element having no thickness(so-called a thin multipole element), fields due to higher-order termsother than the primary term are neglected for the intended purpose ofthe multipole element or merely parasitic factors. However, if thethickness of the multipole element is increased, higher-order termsother than the primary term exhibit effects, which can be used forcorrection of spherical aberration. That is, a multipole element havingthickness t along the optical axis OP creates a field owing tohigher-order terms applicable to correction of spherical aberrationother than the primary term. The thickness t may be different betweenthe individual multipole elements.

An objective lens 114 is mounted downstream of the multipole element112. A pair of transfer lenses 115 consisting of two axisymmetric lensesis mounted between the multipole elements 111 and 112. Similarly,another pair of transfer lenses 115 is mounted between the multipoleelement 112 and the objective lens 114. Each of the transfer lenses 115transfers (projects) a plane (having a magnification of 1 times)equivalent to a plane formed by the multipole element 111 or 112 to thedownstream multipole element 112 or to a coma-free plane of theobjective lens 114.

Plural stages (for example, two stages as shown in FIG. 1) of deflectors116 are mounted between the transfer lenses 115 of each pair to deflectan electron beam 120 (i.e., a charged particle beam) by the use of amagnetic or electric field in two mutually perpendicular directionsperpendicular to the optical axis OP such that some parts of the orbitof the beam 120 are translated.

Generally, axial deviation-induced aberrations in two N-fold astigmaticfields are (N-1)-fold astigmatisms. In particular, in theabove-described configuration, if an axial deviation occurs in the twothree-fold astigmatic fields, a new two-fold astigmatism occurs. Thiscan be confirmed on a sample surface 118. This two-fold astigmatism canbe removed by translation of the parts of the orbit of the electron beam120 performed by the deflectors 116 mounted between the multipoleelements 111 and 112. Consequently, the beam 120 passes on therotationally symmetric axes of the multipole elements 111 and 112, thuscompleting the axial alignment.

A method of making axial alignment of the three stages of multipoleelements is next described by referring to FIG. 2. An aberrationcorrector 10 includes at least three stages of multipole elements 11,12, 13, a pair of transfer lenses 15 mounted between the multipoleelements 11 and 12, another pair of transfer lenses 15 mounted betweenthe multipole elements 12 and 13, a further pair of transfer lenses 15mounted between the multipole element 13 and an objective lens 14, andthree sets of deflectors 16. Each set of deflectors 16 is locatedbetween the transfer lenses 15 of respective one pair. Since themultipole elements 11, 12, 13, the pairs of transfer lenses 15, and thedeflectors 16 are identical in structure and functions with themultipole elements 111, 112, the pairs of transfer lenses 115, and thedeflectors 116, respectively, shown in FIG. 1, their description isomitted.

As described previously, the axial deviation-induced aberration in athree-fold astigmatic field is a two-fold astigmatism. If axialdeviation-induced aberrations occur between the multipole elements 11and 12 and between the multipole elements 12 and 13, the axialdeviation-induced aberrations are combined and appear on the samplesurface 18.

However, unlike the two stages of multipole elements 111 and 112 shownin FIG. 1, the three stages of multipole elements 11, 12, and 13 producetwo axial deviation-induced aberrations of the same order and the sametype (i.e., 2 two-fold astigmatisms). Furthermore, it is impossible todiscern which one of the axial deviation-induced aberrations is due toaxial deviation of the two three-fold astigmatic fields produced eitherby the multipole elements 11 and 12 or by the multipole elements 12 and13. In an attempt to remove these aberrations, it is conceivable toadjust the part of the orbit of the electron beam 20 incident on themultipole element 12 and then to adjust the part of the orbit incidenton the multipole element 13. The first adjustment frequentlysuperimposes a new aberration onto the axial deviation-inducedaberration on the sample surface 18. Therefore, it is difficult toremove the axial deviation-induced aberrations while identifying thetwo-fold astigmatisms.

Accordingly, in the present embodiment, the electron beam is manipulatedsimultaneously between the multipole elements 12, 13, and 14 by makinguse of an optical action occurring by coupling of the 2 two-foldastigmatisms. As described in JP-A-2008-123999, in a case where 2two-fold astigmatic fields are produced, a two-fold astigmatism and anaxisymmetric dispersion occur. The dispersion is the action of aso-called concave lens. These actions are produced by an axial deviationbetween 2 three-fold astigmatic fields as exemplified in the presentembodiment and by an axial deviation between other two N-fold astigmaticfields (N is an integer), as well as by an axial deviation betweentwo-fold astigmatic fields.

In other words, the aforementioned concave lens action vanishessimultaneously with two-fold astigmatisms when the electron beam 20passes on the rotationally symmetric axes of 2 two-fold astigmaticfields. In this case, the concave lens action that reduces the convexlens action of the objective lens vanishes and so the beam is mostoverfocused near the sample surface 18. That is, in the case of thethree stages of multipole elements 11, 12, and 13 shown in FIG. 2, partsof the orbit of the beam 20 are translated relative to the multipoleelements 12 and 13 at the same time to cancel out the two-foldastigmatism produced by an axial deviation between the multipoleelements 11 and 12 and the two-fold astigmatism produced by an axialdeviation between the multipole elements 12 and 13. The beam 20 exitingfrom the multipole element 13 is most overfocused around the samplesurface 18. If the manipulation is stopped at this point, the beam 20will pass on the rotationally symmetric axes of the multipole elements11, 12, and 13, thus achieving an axial alignment.

Operations similar to the foregoing operations are performed in a casewhere there are four or more stages of multipole elements. That is, theparts of the orbit of the electron beam incident on the multipoleelements, respectively, are translated simultaneously so as to cancelout appearing two-fold astigmatisms. The translational motions arestopped when the beam is most overfocused.

A charged particle beam system associated with a first embodiment of thepresent invention is next described by referring to FIGS. 3 and 4.

FIG. 3 schematically shows one example of charged particle beam systemassociated with embodiments of the invention. The charged particle beamsystem, indicated by numeral 30, is a transmission electron microscope,for example, and has an electron gun 31, a first condenser lens 32, theaforementioned aberration corrector 10, a second condenser lens 33, anobjective lens assembly 14 including a sample stage (sample chamber), anintermediate/projector lens system 34, an observational chamber 35, anda power supply 36 for controlling the beam within the electron-opticalcolumn by applying voltages or currents to the optics. An aberrationcorrector and deflectors for axial alignment may be appropriatelyinstalled around the condenser lenses 32 and 33 to correct astigmatismsproduced by the lenses 32 and 33.

Furthermore, the charged particle beam system 30 includes a controller37 that controls the power supply 36 to set the voltages or currentsapplied to the optics including the first and second condenser lenses32, 33 and the aberration corrector 10. Data stored in the storageportion (not shown) within the controller 37 or values entered into theinput portion of the controller 37 by an operator are used as those setvalues. Furthermore, the controller 37 displays an observation imageobtained by the observational chamber 35.

The electron gun 31 produces an electron beam (not shown) in response toapplied voltage and current values. A high voltage of several kV tohundreds of kV is impressed on the gun 31. The emitted electron beam(not shown) is accelerated toward the downstream first condenser lens 32up to a desired energy. The beam is focused by the first condenser lens32. Astigmatism is removed by the aberration corrector 10. Then, thebeam is again focused by the second condenser lens 33. Astigmaticcorrecting elements, deflectors, apertures, and other components areappropriately installed upstream and downstream of the first condenserlens 32 and second condenser lens 33.

After passing through the aberration corrector 10, the beam is furtherfocused in the objective lens assembly 14 and made to impinge on asample placed on the sample stage. The beam transmitted through thesample is magnified by the intermediate/projector lens system 34 andhits a fluorescent screen (not shown) in the observational chamber 35.The sample image (real-space image, diffraction image, or the like)projected onto the fluorescent screen is picked up by an imaging devicesuch as a CCD camera (not shown) and outputted as an image data to thecontroller 37.

FIG. 4 schematically shows another example of the charged particle beamsystem associated with embodiments of the invention. The chargedparticle beam system is indicated by numeral 30′ and is a transmissionelectron microscope that is similar in configuration with the chargedparticle beam system 30 of FIG. 3 except that the aberration corrector10 is installed between the objective lens assembly 14 and theintermediate/projector lens system 34. That is, the beam transmittedthrough the sample is subjected to an aberration correction made by theaberration corrector 10.

An operation for making axial alignment of the electron beam in a mannerassociated with the present embodiment using the aberration corrector 10is next described by referring to FIGS. 2-4. First, the controller 37controls the multipole elements 11, 12, and 13 to produce three-foldastigmatisms. The controller 37 then controls the deflectors 16 todeflect parts of the orbit of the electron beam 20 such that the partsof the orbit hit the multipole elements 12 and 13 at given angles ofincidence.

The controller 37 then controls the deflectors 16 to translate the partsof the orbit of the beam 20 incident on the multipole elements 12 and 13simultaneously in a direction perpendicular to the optical axis OP suchthat two-fold astigmatism produced by an axial deviation between themultipole elements 11 and 12 and two-fold astigmatism produced by anaxial deviation between the multipole elements 12 and 13 cancel out eachother. Because of the translational motions, the angles of incidence ofparts of the orbit of the beam 20 to the multipole elements 12 and 13 donot vary. The controller 37 confirms the focal condition on the samplesurface 18 during the translation motions. When the most overfocusedstate is achieved, the translational motions are stopped. Theoverfocused condition can be checked, for example, by performing knownimage processing on the observed image.

When the electron beam 20 is most overfocused on the sample, the twoastigmatisms are completely removed. The beam 20 is allowed to pass onthe rotationally symmetric axes of the multipole elements 11, 12, and13.

Second Embodiment

A method of making axial alignment of a charged particle beam and acharged particle beam system associated with a second embodiment of theinvention are next described.

The charged particle beam system associated with the present embodimentis similar to the system associated with the first embodiment exceptthat the electron beam 20 is controlled differently than by thecontroller 37 of the first embodiment. Therefore, only the controlprovided by the controller 37 is described below.

In the method of making axial alignment in accordance with the firstembodiment, parts of the orbit of the beam are translated so as tocancel out plural two-fold astigmatisms. A check is made as to whetherthe axial alignment has been achieved from variations of the focalcondition during the translational motions. On the other hand, in thepresent embodiment, hexapole astigmatism that is a higher-orderaberration produced by two adjacent three-fold astigmatic fields isnoticed. As described below, when axial deviations occur among themultipole elements 11, 12, and 13 producing three-fold astigmaticfields, five-fold astigmatism is produced from six-fold astigmatismswhich are produced, respectively, between the multipole elements 11 and12 and between the multipole elements 12 and 13.

The generation of the five-fold astigmatism is now described byreferring back to FIG. 1. It is assumed that the multipole elements 111and 112 have thickness t along the optical axis OP and that theobjective lens 114 has a focal distance of f. Let M be a magnificationprovided by the multipole elements 111, 112 and objective lens 114.Six-fold astigmatism (see non-patent document 3) is given by

$\begin{matrix}{A_{6} = {\frac{{\overset{\sim}{A}}_{3}^{2}{{\overset{\sim}{A}}_{3}}^{2}}{14\mspace{11mu} M^{6}f^{6}}t^{7}}} & (1)\end{matrix}$where Ã₃ is a three-fold astigmatism per unit length produced in each ofthe multipole elements 111 and 112.

Then, it is assumed that the multipole elements 11, 12, and 13 arearranged along the optical axis in three stages as shown in FIG. 2. Inthe same way as the foregoing, each of the multipole elements 11, 12,and 13 creates a three-fold astigmatic field. As described previously,the objective lens assembly 14 of FIG. 2 is the same as the objectivelens 114 of FIG. 1.

When an axial deviation occurs between the multipole elements of theoptical system shown in FIG. 2, the two adjacent multipole elements 11and 12 produce a six-fold astigmatism A₆. Similarly, the adjacentmultipole elements 12 and 13 produce another six-fold astigmatism A₆. Ifthese 6-fold astigmatisms A₆ produce an axial deviation T, its waveaberration is given by

$\begin{matrix}{{{Re}\left\{ {\frac{1}{6}A_{6}{\overset{\_}{\left( {\omega + T} \right)}}^{6}} \right\}} = {{Re}\left\{ {\frac{1}{6}{A_{6}\left( {{\overset{\_}{\omega}}^{6} + {6\;{\overset{\_}{T\;\omega}}^{5}} + {15\;{\overset{\_}{T}}^{2}{\overset{\_}{\omega}}^{4}} + {20\;{\overset{\_}{T}}^{3}{\overset{\_}{\omega}}^{3}} + {15\;{\overset{\_}{T}}^{4}{\overset{\_}{\omega}}^{2}} + {6\;{\overset{\_}{T}}^{5}\overset{\_}{\omega}} + {\overset{\_}{T}}^{6}} \right)}} \right\}}} & (2)\end{matrix}$where ω is a complex angle. The second term of the right side of Eq. (2)contains the fifth power of the complex angle ω, and it can be seen thatit represents a five-fold astigmatism. This is identically equal to

${Re}\left\{ {\frac{1}{5}A_{5}{\overset{\_}{\omega}}^{5}} \right\}$

Therefore, the second term of the right side of Eq. (2) means that afive-fold astigmatism whose amount is 5A₆T is produced. That is, if theaxial deviation T occurs between the 2 six-fold astigmatisms, eachproduced by two adjacent ones of the three stages of multipole elements11, 12, and 13, then the above-described five-fold astigmatism isproduced. Accordingly, in a case where the five-fold astigmatism (or,axial deviation-induced aberration) on the sample surface 18 hasvanished, the electron beam 20 passes on the central axes (rotationallysymmetric axes) of the multipole elements 11-13.

Fundamentally, an axial deviation-induced aberration is produced by thepresence of two adjacent three-fold astigmatic fields accompanied by anaxial deviation. Therefore, the number of the multipole elements (3-foldastigmatic fields) is not limited to three. For example, where fourstages of multipole elements are arranged, a five-fold astigmatism isproduced by axial deviations between three six-fold astigmatisms.Therefore, in this case, when the five-fold astigmatism vanishes on thesample, the charged particle beam passes on the central axes of themultipole elements, in the same way as the foregoing.

Axial alignment of an electron beam by a method associated with thepresent embodiment is next described. First, the controller 37 controlsthe multipole elements 11, 12, and 13 to produce three-fold astigmaticfields. Then, the controller 37 controls the deflectors 16 locatedbetween the multipole elements to deflect parts of the orbit of theelectron beam 20 such that they hit the multipole elements at givenangles of incidence.

The controller 37 controls the deflectors 16 positioned between themultipole elements to translate simultaneously those parts of the orbitof the beam 20 which hit the multipole elements 12 and 13, respectively,to translate these parts of the orbit in a direction perpendicular tothe optical axis OP such that the two-fold astigmatism produced due toaxial deviation of the multipole elements 11 and 12 and the two-foldastigmatism produced due to axial deviation of the multipole elements 12and 13 cancel out each other. Because of the translational motions, theangles of incidence of the beam 20 to the multipole elements 12 and 13do not vary. The controller 37 checks the five-fold astigmatismappearing on the sample surface 18 during the translational motions.When the five-fold astigmatism vanishes, the translational motions arestopped. This five-fold astigmatism can be checked, for example, byperforming known image processing on the observed image.

Where the five-fold astigmatism vanishes on the sample surface 18, theelectron beam 20 is passing on the rotationally symmetric axes of themultipole elements 12-13.

Third Embodiment

A method of making axial alignment of a charged particle beam in amanner associated with a third embodiment of the invention and a chargedparticle beam system for implementing the method are next described.

Referring still to FIGS. 1-3, a charged particle beam system associatedwith the present embodiment has a moving mechanism 19 for moving themultipole elements 11, 12, 13 or at least transfer lens pairs 15 betweenthe multipole elements, in addition to the configuration of the chargedparticle beam system 30 or 30′ of the first or second embodiment.Concomitantly, the controller 37 controls movement made by the movingmechanism 19. The third embodiment is similar to the first and secondembodiments in other respects and so only the control provided by themoving mechanism 19 and controller 37 is described below.

The moving mechanism 19 is mounted, for example, in the electron opticalcolumn (not shown) of the charged particle beam system 30 or 30′, and isa manipulator for independently moving the multipole elements 11, 12, 13or at least the two transfer lens pairs 15 between the multipoleelements in two directions perpendicular to the optical axis OP.

In the first and second embodiments, the aforementioned parts of theorbit of the electron beam 20 are translated to cancel out the two-foldastigmatisms produced between the multipole elements 11, 12, and 13. Inthe present embodiment, instead of translating parts of the orbit of thebeam 20, the multipole elements 11, 12, and 13 are moved independentlyand simultaneously by the use of the moving mechanism 19 such that thetwo-fold astigmatisms cancel out each other. The multipole elements11-13 are moved in directions perpendicular to the optical axis OP.Alternatively, the transfer lens pairs 15 between the multipole elementsmay be moved by the moving mechanism independently and simultaneously.Also, in this case, the transfer lenses 15 are moved in directionsperpendicular to the optical axis OP.

The controller 37 controls the multipole elements 11, 12, and 13 toproduce three-fold astigmatic fields. The controller 37 then controlsthe deflectors 16 to deflect the electron beam 20 such that the beam 20enters the multipole elements at given angles of incidence.

The controller 37 then translates the multipole elements 11, 12, and 13independently and simultaneously by controlling the moving mechanism 19such that the two-fold astigmatism produced by an axial deviationbetween the multipole elements 11 and 12 and the two-fold astigmatismproduced by an axial deviation between the multipole elements 12 and 13cancel out each other. Because of the translational motions, the anglesof incidence of the parts of the orbit of the beam 20 hitting themultipole elements 12 and 13 do not vary. The controller 37 checks thefocal condition or five-fold astigmatism appearing on the sample surface18 during the translational motions. When the beam is most overfocusedor the five-fold astigmatism vanishes, the controller stops thetranslational motions.

As described already in the first and second embodiments, when the beamis most overfocused or the five-fold astigmatism vanishes on the samplesurface 18, the two-fold astigmatisms are completely removed. Theelectron beam 20 is passing on the rotationally symmetric axes of themultipole elements 11, 12, and 13.

In the axial alignment making method and charged particle beam systemassociated with the first through third embodiments, a variation in thefocal state or five-fold astigmatism is utilized by translating parts ofthe orbit of the electron beam between multipole elements, the multipoleelements, or transfer lens pairs in this way. Translational motions aredone such that two-fold astigmatisms, i.e., axial deviation-inducedaberrations, produced between the multipole elements cancel out eachother. Consequently, axial alignment can be made while paying attentionto only the focal state or five-fold astigmatism irrespective of thenumber of stages of multipole elements. Accordingly, the operation formaking axial alignment is facilitated. This can contribute to a decreasein the observation time. Furthermore, the time taken to manipulate thecharged particle beam system is shortened. Hence, the burden on theoperator can be reduced.

The aberration corrector 10 may be installed in an imaging opticalsystem configured including an intermediate lens and a projector lens.Aberration correctors may be installed in both the imaging opticalsystem and in a condenser optical system configured including the firstcondenser lens. In any case, the above-described advantageous effectscan be obtained.

Fourth Embodiment

A method of making axial alignment of a charged particle beam in amanner associated with a fourth embodiment of the invention and acharged particle beam system for implementing the method are described.

In the charged particle beam system associated with the presentembodiment, an aberration corrector 40 shown in FIG. 5 is installedinstead of the aberration corrector 10 of the first embodiment.Accordingly, the system of the fourth embodiment is similar to thecharged particle beam system 30 or 30′ of the first embodiment exceptthat the aberration corrector 40 is installed and that the controller 37controls the electron beam 20 in a different manner. The aberrationcorrector 40 and the control provided by the controller 37 are describedbelow. Description of other configurations are omitted because they areidentical with their counterparts of the first embodiment.

As shown in FIG. 5, the aberration corrector 40 associated with thefourth embodiment has at least three stages of multipole elements 41,42, 43, a condenser lens 51 that is an axisymmetric lens mountedupstream of the multipole element 41 placed in the most front stage,pairs of transfer lenses 45 installed downstream of the multipoleelements 41, 42, and 43, respectively, deflectors 46 mounted upstream ofthe condenser lens 51, and deflectors 47, 48, and 49 mounted between thetransfer lenses 45 of their respective pairs. Each pair of transferlenses 45 consists of two stages of axisymmetric lenses. Each of themultipole elements 41, 42, and 43 produces a three-fold astigmaticfield, and is identical in structure with the multipole elements 11, 12,and 13 described in the first embodiment. The pairs of transfer lenses45 are identical in structure with the pairs of transfer lenses 15described in the first embodiment. The deflectors 46-49 are identical instructure with the deflectors 16 described in the first embodiment.

The condenser lens 51 focuses the electron beam 20, which has beendeflected by the deflectors 46 located upstream of the condenser lens51, toward the multipole element 41.

In the present embodiment, a first deflecting operation consisting ofdeflecting steps (1)-(3) and a second deflecting operation consisting ofa deflection step (4) as described below are simultaneously carried outto achieve axial alignment together with correction of four-foldastigmatism. Angles θ₁-θ₄ given below are made between the optical axisOP and the electron beam 20. In FIG. 5, a counterclockwise direction istaken as a positive direction.

(1) As shown in FIG. 5, the controller 37 controls the deflectors 46 todeflect the electron beam 20 propagating on the optical axis OP onceaway from the axis OP by angle θ₁. Then, the beam 20 is deflected towardthe optical axis OP by angle—θ₂ such that the beam strikes the multipoleelement 41 at an angle.

(2) The controller 37 controls the deflectors 47 to deflect the beam 20exiting from the multipole element 41 at an angle of—θ₂ such that thebeam approaches the optical axis OP at an angle of θ₃ and hits themultipole element 42 at an angle.

(3) The controller 37 controls the deflectors 48 to deflect the beam 20leaving from the multipole element 42 at an angle of θ₃ such that thebeam approaches the optical axis OP at an angle of—θ₄ and strikes themultipole element 43 at an angle.

(4) The controller 37 controls the deflectors 49 to deflect the beam 20going out of the multipole element 43 at an angle of—θ₄ such that thebeam travels on the optical axis OP. The angles θ₁ to 04 are variedsimultaneously in such a way that other first-order, second-order, andthird-order aberrations are not produced.

While the deflecting steps (1)-(4) are being carried out simultaneously,only the four-fold astigmatism on the sample surface 18 varies. Wherethe deflecting steps are stopped when the four-fold astigmatism hasdisappeared, the electron beam 20 passes through the centers P of themultipole elements 41, 42, and 43, the centers being along the opticalaxis OP. That is, correction of the four-fold astigmatism and axialalignment of the beam 20 with the multipole elements 41, 42, and 43 areachieved.

In the combinations of the angles θ₁-θ₄, the polarity (positive ornegative) of each angle may be reversed. For instance, in the deflectingstep (1), if the beam 20 is deflected by an angle of—θ₁, the deflectionangle used thereafter is θ₂.

As described also in the first embodiment, if parts of the orbit of theelectron beam respectively hitting three or more stages of multipoleelements are manipulated separately, various aberrations will appear onthe sample surface 18. Therefore, it is difficult to identifyaberrations used for axial alignment, thereby lengthening theadjustment. In the present embodiment, an axial alignment is made whichuses a variation of four-fold astigmatism by employing simultaneousdeflection of the parts of the orbit of the beam hitting the multipoleelements. Consequently, it is easy to judge whether or not the axialalignment has been achieved. Accordingly, the operation for the axialalignment is facilitated. This can contribute to a decrease in theobservation time. Furthermore, the charged particle beam system can bemanipulated in a reduced time. As a result, the burden on the operatorcan be reduced.

For the sake of illustration, in the description of the presentembodiment, an arrangement of three stages of multipole elements istaken as an example. The number of the multipole elements is notrestricted to three. An axial alignment of four or more stages ofmultipole elements can be made while paying attention to only four-foldastigmatism.

Fifth Embodiment

A method of making axial alignment of a charged particle beam inaccordance with a fifth embodiment of the invention and a chargedparticle beam system for implementing the method are described. Thepresent embodiment is similar to the fourth embodiment except that thedeflecting steps (2) and (3) of the first deflecting operation for theelectron beam 20 are replaced by deflecting steps 2′ and 3′ fortranslating parts of the orbit of the beam relative to two multipoleelements.

(1′) As shown in FIG. 6, the controller 37 controls the deflectors 46 todeflect the electron beam 20 propagating on the optical axis OP suchthat the beam once goes away from the optical axis OP at an angle of θ₁and then to deflect the beam toward the optical axis OP at an angleof—θ₂ and make the beam impinge on the multipole element 41 at an angle.

(2′) The controller 37 controls the deflectors 47 to deflect the beam 20exiting from the multipole element 41 at an angle of—θ₂ in such a waythat the beam moves toward the optical axis OP at an angle of θ₃ andthen to translate a part of the orbit of the beam 20 a distance of r₃ ina direction perpendicular to the optical axis OP such that the beamstrikes the multipole element 42 at an angle.

(3′) The controller 37 controls the deflectors 48 to deflect the beam 20going out of the multipole element 42 at an angle of θ₃ such that thebeam approaches the optical axis OP at an angle of—θ₄ and to translate apart of the orbit of the beam 20 a distance of r₄ in a directionperpendicular to the optical axis OP, thereby making the beam hit themultipole element 43 at an angle.

(4′) The controller 37 controls the deflectors 49 to deflect the beam 20exiting from the multipole element 43 at an angle of—θ₄ such that thebeam propagates on the optical axis OP.

The angles θ₁-θ₄ and distances r₃, r₄ are so set that other first-order,second-order, third-order, and fourth-order aberrations are notproduced, and the angles and distances are varied at the same time.

While the deflections and translational motions are being made at thesame time, only five-fold astigmatism on the sample surface 18 varies.Where the deflections are stopped at the instant when the five-foldastigmatism vanishes, the electron beam 20 passes through the centers Pof the multipole elements 41, 42, and 43 taken along the optical axisOP. That is, correction of the five-fold astigmatism and axial alignmentof the beam 20 with the multipole elements have been achieved.

In this way, in the axial alignment method and charged particle beamsystem associated with the fifth embodiment, variations of the five-foldastigmatism caused by simultaneous deflections and translational motionsof the parts of the orbit of the electron beam hitting the multipoleelements are utilized. Because of the deflections and the simultaneoustranslational motions, only the five-fold astigmatisms vary. Therefore,it is easy to judge whether or not an axial alignment has been achieved.Consequently, in the same way as in the fourth embodiment, it is easy toperform the operation for the axial alignment. This can contribute to adecrease in the observation time. Furthermore, the charged particle beamsystem can be manipulated in a reduced time. Hence, operator's burdencan be reduced.

For convenience of illustration, in the description of the presentembodiment, an arrangement of three stages of multipole elements istaken as an example. The number of stages of multipole elements is notrestricted to three. An axial alignment of four or more stages ofmultipole elements can be made while paying attention to only four-foldastigmatism.

Furthermore, in the present embodiment, an axial alignment can also beattained by reversing the direction of propagation of the electron beam20 and simultaneously effecting the aforementioned deflections andtranslational motions.

Sixth Embodiment

A method of making axial alignment of a charged particle beam in amanner associated with a sixth embodiment of the invention and a chargedparticle beam system for implementing the method are described.

The charged particle beam system associated with the present embodimentis similar in configuration to the charged particle beam system of thefourth embodiment except that three-fold astigmatic fields produced bythe multipole elements 41, 42, and 43 are varied in intensity at thesame time. Description of the configurations of the charged particlebeam system associated with the present embodiment is omitted becausethe configurations are the same as their counterparts of the fourthembodiment, except for the manner in which the multipole elements 41,42, and 43 are controlled by the controller 37.

Each of the three stages of multipole elements 41, 42, and 43 (see FIG.5) associated with the present embodiment is, for example, a dodecapole(12-pole) element that produces a three-fold astigmatic field. In thisoptical system, the first multipole element 41 on the most upstreamside, the second multipole element 42 located at an intermediateposition, and the third multipole element 43 on the most downstream sideproduce three-fold astigmatisms which are herein denoted by A₃₁, A₃₂,and A₃₃, respectively.

H. Rose, Optik, Vol. 85 (1990), pp. 19-24 and H. Haider et al., Nature,vol. 392 (1998), pp. 768-769) state that, when the summation of thesethree-fold astigmatisms A₃₁, A₃₂, and A₃₃ becomes equal to 0 under thecondition where the summation of any two of them does not cancel thesummation of other two of them, the whole three-fold astigmatism isnull.

FIGS. 7A-7B and 8A-8D show vectorial representations of three-foldfields. In the case of vectorial representations of a three-fold field,it is customary to use values that are three times greater than actualrotational relations of angles in the three-fold field within an opticalor physical multipole element. According to this custom, when threestages of three-fold fields optically cancel out each other and thethree-fold astigmatism becomes null, if vectorial representations areused, the combination of three vectors is geometrically null, which iseasy to understand.

The main aberration remaining under the condition where the wholethree-fold astigmatism is 0 is spherical aberration. Accordingly, if theintensity distribution of the three-fold astigmatic fields produced bythe multipole elements 41, 42, and 43 is so varied that the summation ofthe 3 three-fold astigmatisms A₃₁, A₃₂, and A₃₃ is kept at 0 at alltimes, variations of the spherical aberration are prevalent. In FIGS. 7Aand 7B, variations of the three-fold astigmatisms A₃₁, A₃₂, and A₃₃ arerepresented vectorially.

In particular, the distributions (amplitudes) of the three-foldastigmatic fields produced by the multipole elements 41-43 are variedwhile the summation of the 3 three-fold astigmatisms A₃₁, A₃₂, and A₃₃is maintained at 0 by controlling the three-fold astigmatic fieldsproduced by the multipole elements 41-43 by the use of the controller37. If the variations are brought to a stop when the sphericalaberration in the objective lens is reduced maximally, an axialalignment in which the whole spherical aberration has been mostcorrected can be accomplished.

If the three-fold astigmatic fields produced by the three stages ofmultipole elements are manipulated separately as in the related art,varied aberrations appear on the sample surface 18. This makes itdifficult to identify aberrations used for axial alignment and lengthensthe adjustment. In the present embodiment, an axial alignment is madeusing variations of spherical aberration caused by simultaneousadjustment of the three-fold astigmatic fields produced by the multipoleelements. Therefore, it is easy to judge whether or not the axialalignment has been achieved. Accordingly, the operation for the axialalignment is facilitated. This can contribute to a decrease in theobservation time. Furthermore, the charged particle beam system ismanipulated in reduced time. Consequently, operator's burden can bealleviated.

Where the intensities or rotational angles of three-fold fields are notset appropriately, three-lobe aberration that is a fourth-orderaberration remains as shown in FIGS. 8A-8D. FIGS. 8A-8B are schematicdiagrams showing states in which the phase angle difference between thethree-fold astigmatism A₃₁ produced by the first multipole element 41and the three-fold astigmatism A₃₃ produced by the third multipoleelement 43 is not (70.5/3)°. FIGS. 8C-8D are schematic diagrams showingstates in which the phase angle difference between the three-foldastigmatism A₃₁ produced by the first multipole element 41 and thethree-fold astigmatism A₃₃ produced by the third multipole element 43 is(70.5/3)° but the aberrations A₃₁ and A₃₃ are not equal in intensity.

When the summation of the three-fold astigmatisms A₃₁, A₃₂, and A₃₃produced by the multipole elements 41, 42, and 43 is 0 and the phaseangle difference between the three-fold astigmatism A₃₁ produced by thefirst multipole element 41 and the three-fold astigmatism A₃₃ producedby the third multipole element 43 deviates from (70.5/3)° as shown inFIGS. 8A and 8B, three-lobe aberration remains. On the other hand, whenthe three-fold fields are so set that the summation of the three-foldastigmatisms A₃₁, A₃₂, and A₃₃ produced by the multipole elements 41-43is 0 and, at the same time, the phase angle difference between thethree-fold astigmatism A₃₁ produced by the first multipole element 41and the three-fold astigmatism A₃₃ produced by the third multipoleelement 43 is (70.5/3)° but the astigmatisms A₃₁ and A₃₃ are not equalin intensity as shown in FIGS. 8C and 8D, three-lobe aberration remains.Accordingly, the distribution of the three-fold astigmatic fieldsproduced by the multipole elements 41, 42, and 43 is so varied that theastigmatisms A₃₁ and A₃₃ are equal in intensity and that the phase angledifference between the three-fold astigmatism A₃₁ produced by the firstmultipole element 41 and the three-fold astigmatism A₃₃ produced by thethird multipole element 43 is approximately (70.5/3)°. If this variationis brought to a stop when the spherical aberration in the objective lensor three-lobe aberration has been reduced to a minimum, an axialalignment in which the two aberrations have been most corrected as awhole can be achieved.

While embodiments of the present invention have been described so far,charged particle beam systems of the embodiments are not restricted totransmission electron microscopes. The charged particle beam systems ofthe embodiments can also be applied to scanning electron microscopes,scanning transmission electron microscopes, ion microscopes, focused ionmicroscopes, and similar other instruments.

The invention claimed is:
 1. A method of making axial alignment of acharged particle beam in a charged particle beam system, said methodcomprising the steps of: producing at least 3 three-fold astigmaticfields; producing a pair of rotationally symmetric fields between anyadjacent two of the astigmatic fields to transfer planes equivalent toplanes formed by the astigmatic fields to their downstream astigmaticfields; and simultaneously performing first deflections to cause partsof the orbit of the charged particle beam to enter the three-foldastigmatic fields obliquely.
 2. The method of making axial alignment ofa charged particle beam as set forth in claim 1, further comprising thestep of performing a second deflection simultaneously with said firstdeflections, the second deflection operating to cause the chargedparticle beam exiting from the three-fold astigmatic field distributedon the most downstream side of said at least 3 three-fold astigmaticfields to propagate on an optical axis.
 3. The method of making axialalignment of a charged particle beam as set forth in claim 1, whereinsaid first deflections include translating the charged particle beamrelative to other than the three-fold astigmatic field distributed onthe most upstream or downstream side of the three-fold astigmaticfields.
 4. The method of making axial alignment of a charged particlebeam as set forth in claim 3, wherein said first deflections includetranslating the charged particle beam relative to other than thethree-fold astigmatic field distributed on the most upstream ordownstream side of the three-fold astigmatic fields.
 5. A chargedparticle beam system comprising: at least three stages of multipoleelements for producing three-fold astigmatic fields; pairs of transferlenses mounted between successive ones of the multipole elements suchthat each pair of transfer lenses produces a pair of rotationallysymmetric fields to transfer planes equivalent to planes formed by themultipole elements to their downstream multipole elements; and firstdeflectors mounted between the transfer lenses of each pair and mountedon the upstream side of the at least three stages of multipole elementsand operating to deflect parts of the orbit of the charged particle beamsimultaneously so as to hit the optical axis obliquely.
 6. The chargedparticle beam system as set forth in claim 5, wherein said firstdeflectors excluding the deflectors mounted upstream of the multipoleelement located on the most upstream or most downstream side operate totranslate parts of the orbit of the charged particle beam as well as todeflect the beam as mentioned previously.
 7. The charged particle beamsystem as set forth in claim 5, further comprising second deflectors fordeflecting the charged particle beam exiting from the multipole elementlocated on the most downstream side of said at least three stages ofmultipole elements to cause the beam to propagate on the optical axis,and wherein the deflections by the first and second deflectors are madeat the same time.
 8. The charged particle beam system as set forth inclaim 6, further comprising second deflectors for deflecting the chargedparticle beam exiting from the multipole element located on the mostdownstream side of said at least three stages of multipole elements tocause the beam to propagate on the optical axis, and wherein thedeflections by the first and second deflectors are made at the sametime.